Producing electronic devices—and especially Integrated Circuits (ICs)—is a very complex task, which involves several phases. In general terms, starting from an IC architectural design, the required electronic circuit blocks are designed and simulated by means of CAD tools; then, a physical layout of several different IC layers is defined, and corresponding photolithographic masks (necessary for the planar technology manufacturing process) are fabricated. Once the masks are released, prototypes of the IC are created, and a testing phase is carried out to check whether the IC correctly performs the intended tasks and satisfies the design requirements.
However, during the actual manufacturing of the IC, there are a number of functional parameters (voltages, currents, time delays, resistance and capacitance values) that, despite careful design and simulation, are subjected to variations of their actual values with respect to the intended, expected ones. This is mainly due to the practical and, in some respects even theoretical, impossibility of eliminating fluctuations in several parameters of the manufacturing process (e.g., dopant species concentrations, alignment of masks, and a number of other variables).
As a consequence to these unavoidable, statistical variations in their functional parameters, several of the manufactured ICs may have to be discarded because they do not respect the intended specifications.
In order to avoid this, which could severely impact the manufacturing process yield, IC designers usually provide trimming structures in the ICs, being adapted to implement a post-manufacturing correction of at least the most critical functional parameters.
Several ways are possible for implementing the trimming process of these functional parameters, such as the provision of fuses to be selectively burnt. A more usual and preferred way calls for providing (trimming) configuration registers, usually programmable electrically and capable of retaining information even in absence of power supply. By storing prescribed configuration codes in these registers, different operating configurations for selected circuit blocks can be achieved, so as to correct the desired functional parameters at will.
Typically, this kind of trimming process is performed during the testing phase of the IC, and it can be expediently exploited not only for ensuring that a specific IC satisfies the desired requirements, but also as investigation means; in this way, IC designers and process engineers can refine the design and the process flow with the aim at coming to a consolidated product.
However, the trimming process described above is quite time consuming, since it requires measuring and accordingly correcting each functional parameter of interest individually. Moreover, the measuring of specific functional parameters may be difficult, if not impossible; for example, when a functional parameter is not directly available outside the IC it is necessary to provide additional circuitry dedicated to this purpose (which additional circuitry increases the size and the complexity of the whole IC).
A solution for alleviating the above-described drawbacks in a very specific situation is disclosed in the European Patent Application EP 1591858. Particularly, this document relates to the trimming of reference-voltage generators (for example, of the band-gap type). In this case, the above-described measuring and correcting operations are applied to a single reference-voltage generator only; the other reference-voltage generators are corrected in a proportional way (exploiting the fact that the fluctuations in the manufacturing process affect all the reference-voltage generators in a similar way). In this way, the trimming process of the reference-voltage generators is simplified.
However, the proposed solution is not of general applicability. Particularly, this technique does not solve the problem of measuring other functional parameters; as mentioned above, this problem is particular acute when the functional parameters are not directly available outside the IC.
Moreover, the document EP 1591858 does not cope with the additional problem of trimming the functional parameters with different accuracy. Indeed, in the solution disclosed in this document to each correction applied to the (measured) reference voltage corresponds a proportional correction for each other (non-measured) reference voltage; as a consequence, all the reference voltage generators are always trimmed in the same way. Therefore, it is not possible to discriminate functional parameters requiring different trimming processes (for example, because their accuracy is more or less critical for the operation of the IC).